Semiconductor lasers are used in various application fields for the reason that the semiconductor laser is able to emit coherent laser light with high directivity, and that it is small in size and inexpensive.
The semiconductor laser is constituted by two mirror surfaces positioned to face each other, and a waveguide disposed between the two mirror surfaces. When a current is injected into the semiconductor laser, photons are generated in the waveguide. The generated photons are reflected by the two mirror surfaces to repeatedly reciprocate inside the waveguide such that the number of photons increases in a chain relation and the photons eventually reach a resonant state. There is a difference in reflectance between the two mirror surfaces, and stronger laser light is emitted from the mirror surface having a lower reflectance (higher transmittance). Generally, the stronger laser light is used in various application fields.
On the other hand, laser light having a comparatively low output power is also emitted from the mirror surface having a higher reflectance (lower transmittance). The weaker laser light is generally not used at all or just used as reference information for an output power of the laser light that is emitted from the mirror surface having the lower reflectance. In the latter case, the laser light emitted from the mirror surface having the higher reflectance is monitored by employing a photodiode.
In order to utilize the laser light emitted from the semiconductor laser in various application fields, it is very important to know at least one environmental parameter (e.g., temperature, humidity, or gas concentration), which represents the state of an environment around the semiconductor laser.
Patent Literature (PTL) 1, for example, discloses a technique of predicting the degradation status of a semiconductor laser or avoiding an abrupt failure by continuously monitoring humidity inside a container with a humidity sensor disposed within the container in which an encapsulated semiconductor laser is also disposed.
PTLs 2 to 4 disclose techniques of determining whether dew condensation occurs on a semiconductor laser, or avoiding the occurrence of dew condensation by monitoring temperature and humidity around the semiconductor laser or around an apparatus onto which the semiconductor laser is mounted, with a temperature sensor and a humidity sensor, respectively.